An interferometer of the Michelson type splits an input light beam into a reflected beam and a transmitted beam using a beam splitter. Each split beam travels along its own path to a return mirror, which deflects it back to the beam to splitter along the same path. One of the return mirrors is stationary, while the other is movable, typically along a linear path between two limits equidistant from a datum position. At the beam splitter, the return split beams recombine along a common output path leading to a photodetector.
If the movable mirror is at its datum position, the optical path of the two split beams is the same, so that when those split beams return to the beam splitter they constructively interfere. This results in a large signal produced at the photodetector, known as a center burst. If the movable mirror is shifted towards the incoming split beam, the optical path of that beam decreases and, conversely, if the mirror is moved away the optical path is increased. Thus, as the movable mirror is moved from one limit to another, two complete series of optical path difference values of opposite signs are generated. This travel is referred to as an optical path difference (OPD) scan. The output signal of a photodetector during an OPD scan is a series of superimposed electrical sine waves of different frequencies and amplitudes. This signal is known as an interferogram.
An interferometer also includes a reference light source, typically a laser, which is used to measure the optical path difference (OPD). The reference fringes created during an OPD scan are sensed by a photodetector, which generates a reference fringe signal as a sine wave.
In carrying out an analysis of a sample, an interferometer executes a number of scans, sweeping forward and backwards through the center burst of the infrared interferogram, generating a series of analog-to-digital converter (ADC) readings during part of each scan. The reference fringe signal is used to determine the exact times at which the ADC in the interferogram channel is read, in order to build up a sampled interferogram with constant optical path difference intervals.
In practice, changes in scan direction may occur at slightly different optical path difference values in different sweeps. Lack of detailed knowledge about is where reversals occur in a fringe waveform leads to uncertainty in the absolute optical path difference of points read by the ADC in subsequent scans. If the position of the optical path difference varies by a few microns between each scan, this significantly affects the accuracy of added interferograms and, consequently, affect the quality of the transformed spectra.
One system, as an example, is the cross-track infrared sounder (CrIS) system, which is a Michelson interferometer that measures with high resolution and high spectral accuracy the emission of infrared radiation from the atmosphere in three bands in the spectral range from 3.9 to 15.4 μm (650-2250 cm−1). The core of the instrument is a Fourier transform spectrometer, which measures in one sweep the spectral features of the atmosphere. The spectrometer transforms the incoming spectral radiance, i.e. the spectrum, into a modulated signal, the interferogram, where all infrared wavenumbers in the band of interest are present simultaneously. The output from the spectrometer includes one such interferogram for each observed scene.
The CrIS system includes a spaceborne sensor and ground-based algorithms, as shown in FIG. 1. As shown, CrIS system 10 includes space segment 12 and ground segment 14. A sensor 18 resides in spacecraft (S/C) 16 and obtains raw unprocessed interferograms that are sampled from any of nine different field-of-views (FOVs) and from any of three IR spectral bands. The sensor 18 may measure the following three types of scenes: (a) the spectral radiance of the earth and its atmosphere, (b) the spectral radiance of deep space, and (c) the spectral radiance of the instrument itself as compared to a known blackbody source.
Of all the measurement types listed, only the spectra of the atmosphere contains the desired scientific information. All other measurements are characterization measurements for calibration of sensor 18. Using the results from these characterization measurements, a calibration procedure may then be applied to the scene measurements of the atmosphere.
The ground segment 14 includes command, control and communications system 20 to spacecraft 16 and desired algorithms, such as SDR algorithms 22 and EDR algorithms 24. The SDR algorithms 22 are required to transform raw instrument records (RDRs) into sensor data records (SDRs), the latter is being essentially the calibrated spectra. The SDRs are subsequently transformed into environmental data records (EDRs) by EDR algorithms 24.
Referring next to FIG. 2, there is shown more detail of space segment 12. The interferometer 30 observes three different types of scenes: (1) earth scene (ES), deep space (DS) and internal calibration target (ICT) scenes. After passage of the observed scene through detector 32, amplifier 33 and ADC 34, the real function of the interferogram, I(x), is produced. Noise spike correction of the raw interferogram data may be accomplished by impulse noise clipper 35. The spikes may be present anywhere in the interferogram, near or far from the ZPD. Performing filtering and decimation on the raw interferograms, as shown, may provide more effective results than performing the same via software on the ground. The clipped signal may be filtered by a 255 tap FIR, designated as 36. The output signal from the FIR may be a complex interferogram, including real and imaginary functions. Next, the output signal is bit-trimmed (module 37) and packet encoded (module 38) and downlinked to the ground station as raw data records (RDRs) 39.
As an example, using the CrIS system, the total number of raw sampling points corresponding to the OPD sweep is shown, in Table 1, below. Measured raw data points are then filtered and decimated, as shown, to lower the transfer data rate. The decimation factor used in each band depends on the bandwidth. A set of nominal spectral channel wavenumbers may be provided as an output. The number of output bins or channels is also shown, as an example.
TABLE 1CrIS signal dimensions (an example)OPDDecimationDecimatedOutputBandsamplesfactorpointsbinsLW20 73624864713MW10 56020528433SW 5 20026200159
The CrIS instrument observes the ground with an instantaneous FOV which maps to a nadir footprint of 14 km on the ground, from an altitude of 833 km. The field of regard (FOR) of the instrument is shown in FIG. 3.
A typical CrIS scan sequence may include 34 interferometer sweeps, as shown in FIG. 4, including 30 earth scenes, plus 2 deep space and 2 ICT measurements (these numbers include both forward and reverse sweeps). One scan of the CrIS sensor takes about 8 seconds. The sensor may perform a new measurement (sweep) every 200 ms (7 ms for pointing and 33 ms for repositioning). A new cycle (scan) is repeated every 8 seconds. Each scan includes 918 interferograms.
The two calibration measurements (DS and ICT) are performed once every 8 seconds, in order to account for changing self-emission of the instrument due to temperature variations in orbit.
For a complete description of the CrIS system and its various algorithms, one may refer to the following web document, which is incorporated herein by reference in its entirety:
http://eic.ipo.noaa.gov/IPOarchive/SCI/atbd/BOM-CrIS-00672-SDR-ATBD.pdf